Speeds of fans used to dissipate heat in a system are often controlled by Pulse Width Modulation (PWM) circuits. Usually, a heat dissipation system includes a fan controlled by a rotation control unit, a speed control circuit receiving a PWM signal produced by a rotation control unit and converting the PWM signal to a voltage signal. A motor of the fan receives the voltage signal and then adjusts its speed based on the voltage signal. The voltage signal is in a proportional relationship to the speed of the motor.
In use, however, the described system provides no information regarding the temperature of the system, and speed of the fan cannot be adjusted based on the temperature or user intention. Accordingly, when the speed of the motor automatically increases, noise produced by the high speed is inevitable. Further, in unusual situations the PWM speed control circuit may fail to control the speed of the fan normally, with no user-based remedy available.